Electrolytic treatment of material

ABSTRACT

A method of treating a body of conducting material electrolytically which comprises the steps of placing the material to be treated in an aqueous oxidizing electrolyte, passing an electric current through the electrolyte and the material to be treated and sparging the electrolyte with gas. The material to be treated is placed in a basket comprising a conducting frame having an insulating container fitted therein, the basket being insertable in and removable from the electrolyte as desired. The insulating container is removable from the conducting frame, the frame having retaining means for receiving the insulating container. The container includes a base having perforations to allow electrolyte to contact the material to be treated. The electrolyte is sparged with gas in the region where the material to be treated is in contact with it, the gas being supplied by a plurality of pipes (21, 22) extending across the electrolyte at a location beneath the perforated base of the insulating container. The pipes each have a plurality of gas outlet holes directed towards the material to be treated.

The present invention relates to the electrolytic treatment of material.In particular, it relates to apparatus and a method for treating a bodyof conducting material.

The use of gas bubbling or sparging in electrochemical process is known.Examples of such processes are disclosed inn Patent Specifications GB1091157, GB 1392705, GB 2182259A, U.S. Pat. No. 3,875,041 and U.S. Pat.No. 4,263,120. However, in the constructions described the bubbling orsparging is carried out in a controlled manner at electrode surfaces inorder to promote processes such as electrodeposition or electrolyticetching.

Applicants' GB 2269601A describes a method of treating scrap graphitecontaminated with metal electrolytically whereby graphite and metal maybe separated and treated or disposed of in different ways. The use ofsparging is proposed.

However, GB 2269601A does not envisage a practical arrangement forcarrying out the sparging and it is therefore an object of the presentinvention to provide such a practical arrangement. The present inventionprovides an apparatus and method for electrolytically treating bodies ofconducting material such as graphite in a manner similar to thatdescribed in GB 2269601A.

According to the present invention there is provided a method oftreating a body of conducting material electrolytically which comprisesthe steps of placing the material to be treated in an aqueous oxidisingelectrolyte, passing an electric current through the electrolyte and thematerial to be treated and sparging the electrolyte with gas,characterised in that the material to be treated is placed in a basketcomprising a conducting frame having an insulating container fittedtherein, the basket being insertable in and removable from theelectrolyte as desired, wherein the insulating container is removablefrom the conducting frame, the frame having retaining means forreceiving the insulating container, the container including a basehaving perforations to allow electrolyte to contact the material to betreated and wherein the electrolyte is sparged with gas in the regionwhere the material to be treated is in contact with it, the gas beingsupplied by a plurality of pipes extending across the electrolyte at alocation beneath the perforated base of the insulating container, thepipes each having a plurality of gas outlet holes directed towards thematerial to be treated.

Sparging of the electrolyte in the manner described allows a substantialsurface area of the electrolyte around the material to be treated to besparged simultaneously.

The material to be treated may comprise a porous mass. It may comprisescrap graphite to be separated from metal adhered thereto. The metal mayfor example comprise one or more hazardous, eg toxic or radioactive,elements such as uranium and or plutonium. Such elements may be presentin compound form, eg as oxides.

The electric current may be a directional current.

The conducting material to be treated, eg graphite body, disintegratesin the electrolyte and any metal present dissolves at an acceleratedrate under the influence of the electric current. The metal can alsobreak off from the graphite and may dissolve over a longer time periodin the electrolyte. The graphite so treated may therefore be separatedby filtering and washing. Where the graphite has been contaminated withuranium and/or plutonium the separation by this process is sufficientlysuccessful to allow the graphite to be disposed of in a conventionalmanner rather than by special means required for hazardous, radioactivematerials.

Where scrap graphite is treated by the method of the present inventionthe scrap graphite may contain less than 40 percent, in most cases lessthan 10 percent by weight, eg from 2 to 6 percent by weight ofcontaminant metal so that the metal is a minor by-product to theseparation process (in terms of its quantity).

The electrolyte is desirably a strong acid, eg nitric and/or sulphuricacid. Its concentration is preferably in the range 5 to 70 percent byweight of acid: aqueous solution. In general, the process works morerapidly as the concentration of the acid increases. The process speedalso increases with the further assistance of (a) an elevatedelectrolyte temperature, eg 30 to 80 degrees Celsius; also with (b)mechanical agitation or stirring of the electrolyte and also with (c) anincrease in applied electric current or (d) input of additional energyfrom other sources, eg ultrasonic devices.

The mean applied electric current needs to be greater than the minimumcurrent required for the reaction, which is typically 10 milliamps percm².

Where additional mechanical stirring of the electrolyte is applied thismay be the use of a conventional paddle or agitator. Alternatively,ultrasonic stirring may be used.

The electrolytic system containing the electrolyte may comprise an acidbath into which the material to be treated is placed. The material maybe contained in a basket which may be insertable in and removable fromthe electrolyte as desired. The basket may conveniently comprise aconducting, eg metal, frame having an insulating container fittedtherein. The insulating container, which may for example be made ofplastics material, may be fixed to or removable from the metal frame.The container may itself be made up of individual insulating boardsfitted together. The frame may contain a receiving means, eg ledge forreceiving the container at a height above the base of the frame.

Desirably, the said pipes which provide delivery of the sparging gas arelocated beneath the said insulating container. The purpose of the saidinsulating container is to hold a charge of material to be treated, eg amass of scrap graphite whist maintaining electrical insulation betweenan electrical terminal applied to the charge of material and a terminalof opposite polarity in the electrical cell. The said containerdesirably includes a base having perforations, slits or holes therein toallow electrolyte in the bath to contact the material to be treated.Where a conducting frame is employed to hold the said insulatingcontainer the conducting frame may have a plurality of sites, eg tabshaving holes therethrough, at which electrical terminals all of the samepolarity may be connected, eg by fasteners such as nuts and bolts to theframe. A plurality of conducting rods or bars, eg made of stainlesssteel, may be connected to the said frame beneath the base of the saidinsulating container. The rods or bars which may run parallel to oneanother may extend in use through the electrolyte at a level below thesaid gas sparging pipes. The said rods or bars provide a multipleelectrode structure in the electrolyte beneath the said container.

Desirably, the gas sparging pipes run at an angle to, preferably about90 degrees to the said rods or bars.

A further plurality of gas sparging pipes may be provided beneath thesaid rods or bars whereby in use gas delivered thereby may be applied tothe rods or bars.

We have found that sparging of the electrolyte by gas delivered by thefirst mentioned sparging pipes in the manner described above has thefollowing benefits. Firstly, it helps remove saturated gases, such asNO₂ from HNO₃, which cause slowing down of the electrochemical reaction.Secondly, it provides mechanical energy to help the treated materialbreak up. Thirdly, it frees grains of the broken up treated material.Fourthly, it mixes the electrolyte solution so that the electrolyte incontact with the material to be treated is kept in fresh supply. Therate of delivery of the sparging gas may be chosen by suitableexperimentation. If the rate is too great the current through the cellfalls and this fall can be measured by a suitable meter.

Where a further supply of sparging gas is provided beneath the said rodsor bars which are negative electrodes the gas is desirably oxygen or airwhich promotes the conversion of product formed at the negativeelectrodes eg reconversion of HNO₃ from HNO₂.

The present invention provides a method for recovering scrap conductingmaterial, eg scrap graphite, and has all of the advantages described inGB 2269601A.

Where an insertable basket including an insulating container having aperforated base is employed to contain material to be treated in anelectrolyte bath, the positive electrical terminal may comprise one ormore blocks placed on the mass of material. Each block may be made ofgraphite and may be adapted, eg by having an internal screw thread, toreceive a member connected to a conducting cable. Alternatively, or inaddition, the positive electrode for applying electric current may beprovided by one or more blocks of metal, eg stainless steel, in contactwith the material to be treated and/or by a collar of metal, egstainless steel, inside the basket, eg slidably located against theinner wall thereof, in contact with the material to be treated.Desirably, the electrolyte reaches a level in the said insulatingcontainer which is between the top and bottom levels of the material tobe treated whereby the positive electrical terminal or contact, eggraphite or metal block(s), to the material to be treated is keptoutside the electrolyte. This reduces the possibility of corrosion ofthe said contact. In the said arrangement electrical current flows (frompositive to negative) around the circuit comprising in turn theelectrical contact to the material to be treated; the material to betreated; the electrolyte; the rods or bars; the frame of the basket andthe negative terminals connected to the frame.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described by way ofexample with reference to the accompanying drawings, in which:

FIG. 1 is a front view of a basket structure embodying the presentinvention;

FIG. 2 is an end view of part of the basket structure shown in FIG. 1;

FIG. 3 is a perspective view in the direction X shown in FIGS. 1 and 2.

FIG. 4 is a perspective view in the direction Y shown in FIGS. 1 and 2of an insert for the basket structure shown in FIGS. 1 and 2.

As shown in FIGS. 1 to 3 a basket structure comprises a rectangularframe 1 made of metal, eg stainless steel. The frame 1 comprises at itsfront face or side as shown in FIG. 1 flat elongate, upright plates 3a,5a having flat elongate cross plates 7a, 9a, 11a, 13a and 15a extendingbetween them at right angles thereto. Similarly, the frame 1 comprisesas its end face as shown in FIG. 2 flat, elongate upright plates 3b and5b having flat elongate, cross plates 7b, 9b, 11b, 13b and 15b extendingbetween them at right angles thereto.

The face of the frame 1 opposite to the front face containing the plates3a and 5a comprises (in a manner similar to the front face) flatelongate upright plates (not shown) 3c, 5c having flat, elongate crossplates (not shown), 7c, 9c, 11c, 13c and 15c extending between them atright angles thereto. Likewise, the face of the frame 1 opposite to theface containing the plates 3b, 5b comprises (in a similar manner) flatelongate upright plates (not shown) 3d, 5d having flat elongate crossplates (not shown) 7d, 9d, 11d, 13d and 15d extending between them atright angles thereto.

The plates 3a and 5b, the plates 5a and 3d, the plate 5d and 3c and theplates 5c and 3b are respectively joined together in pairs each pairforming a unitary upright corner member 4 having its respective platesforming a right angle to one another. Thus, the frame 1 has four uprightcorner members 4 as shown in FIG. 3.

The upper cross plates on each side or face of the frame 1 viz each ofthe plates 7a, 7b, 7c, 7d has a plurality of tabs 15 upwardly projectingtherefrom. The tabs 15, which may comprise metal plates welded to theupper cross plates, are provided with holes 17. A nut and bolt (notshown) are attached together through each hole 17 to provide a means forsecuring an electrical terminal to the frame 1 at each hole 17 in amanner similar to that commonly employed for car battery terminals.Thus, a plurality of terminal sites are provided around the top of theframe 1 at the holes 17.

A ledge 19 projects inwardly from the cross plates 9a, 9b, 9c and 9d ofthe frame 1. The ledge 19 may for example be a metal ledge welded to thecross plates 9a, 9b, 9c and 9d. The ledge 19 extends continuously allaround the inner boundary of the frame 1 formed by the plates 9a, 9b, 9cand 9d.

A plurality of gas pipes 21 is fitted to run between and to be supportedby the plates 11a 11c. The pipes 21 also extend between gas pipes 23a,23b (23b not shown) running parallel to the plates 9a, 9c. The pipes 21are joined at their respective ends to the pipes 23a, 23b so that gasfrom a common source (not shown) fitted to the pipe 23a may be deliveredalong the pipe 23a and the pipes 21 and likewise, a plurality of gaspipes 22 parallel to and below the pipes 21 are fitted to run betweenand to be supported by the plates 15a, 15c. The pipes 22 are alsoextended between and are joined to gas pipes 24a, 24b (24b not shown)running parallel to the pipes 23a, 23b. The pipes 22 are joined at theirrespective ends at the pipes 24a, 24b so that gas from a common sourcefitted to the pipe 24a may be delivered along the pipe 24a and the pipes22 and extracted from the pipe 24b. Each of the pipes 21 and 22 has aseries of small diameter gas outlet holes (not shown) provided in itsupper surface.

A plurality of conducting rods 25, eg made of stainless steel, extendbelow gas pipes 21 and above the gas pipes 22 between the plates 13b and13d. The rods 25 run at right angles to the pipes 21 and 22.

FIG. 4 shows a view toward one of the corner members 4 from inside theframe 1. A board 27 made of plastics material is inserted inside theframe 1 and is fitted to abut against and to be supported by the ledge19. The board 27 has parallel slits 29 formed through its thickness toextend between its upper and lower faces. Four further boards 31a, 31b,31c, 31d (31c, 31d not shown) each made of plastics material are fittedinside the frame 1 each in an upright position adjacent to therespective sides of the frame 1. The ends of the boards 31a, 31b, 31c,31d form push fits together and their lower edges abut against the board27 whereby the boards 27, 31a, 31b, 31c and 31d form an open plasticsbox structure 33 having no gaps apart from the slits 29 which formperforations in the base board 27.

In use, conducting material to be treated, eg metal contaminatedgraphite, (not shown) is placed inside the box structure 33. Thematerial to be treated does not reach the top of the box structure 33 sothat the material does not touch the metal of the frame 1. A pluralityof electrical terminals (not shown) are fitted to the frame 1 at theholes 17 in the manner described above. The basket comprising the frame1 including the box structure 33 and charge of material to be treated isthen lowered into a tank containing strong acid, eg concentrated nitricacid. The acid reaches an intermediate level indicated by broken line Lin FIGS. 1 and 2. Finally, one or more heavy conducting, eg graphite,blocks (not shown) are placed on top of the material to be treated abovelevel L whereby the blocks do not make contact with the acid. Each ofthe blocks has an electrical terminal attached thereto. For example,each block may be tapped to receive a screw threaded member attached toa heavy duty conductor.

A direct voltage is applied between (a) the terminals attached to theframe 1 at the holes 17 which are arranged to be negative terminals and(b) the terminal or terminals attached to the heavy block or blockswhich are arranged to be positive. Electrical current thereby flows inthe electrical circuit comprising in turn (from positive to negative)the heavy block(s); the material to be treated; the acid electrolyte,the conducting rods, the frame 1 and the terminals at the holes 17. Thebox structure 33 maintains insulation between the positive terminalconnected to the material to be treated and the negative structureincluding the frame 1.

Gas, eg air or nitrogen, is admitted along the pipes 21 via the pipe 23aand along the pipes 22 via the pipe 24a and causes sparging in the acidin the regions where the gas enters the electrolyte through the holes.The sparging from the pipes 21 causes stirring of the electrolytethroughout a wide region of the acid and material to be treated insidethe box structure 33 by passage of the sparging gas through the openingsprovided by the slits 29. The sparging from the pipes 22 providescleaning of the negative rods 25 inside the electrolyte.

The material to be treated is broken down and consumed in the acid. Thebox structure 33 is recharged before the level falls below theelectrolyte level L, so that the heavy blocks (providing terminals)remain outside the electrolyte. Using a basket structure as shown in theaccompanying drawings in a tank of acid (not shown) forming anelectrolytic cell as described the following experimental examples werecarried out to demonstrate the benefit of employing sparging of theelectrolyte in the treatment of solid conducting material, egcontaminated graphite.

EXAMPLE 1

A basket structure as shown in FIGS. 1 to 3 containing an insert boxstructure 33 as in FIG. 4 was charged with scrap graphite in the mannerdescribed above and incorporated in an electrolytic bath containingnitric acid. An electrical current of 250 amps was passed through thecell containing the scrap graphite for 24 hours. The scrap graphite massdisintegrated and fell through the openings provided by the slits 29 inthe structure 33 at a rate of 400 grammes per hour. The experiment wasrepeated under similar conditions but with the scrap graphite andelectrolyte in the structure 33 sparged with air. The graphite in thiscase fell through the slits 29 at a rate of 1300 grammes per hour.

The whole experiment was repeated several times and each time theprovision of sparging improved the fall out rate of graphite grainscompared with the case with no sparging applied.

EXAMPLE 2

A basket structure as shown in FIGS. 1 to 3 containing an insert boxstructure 33 as shown in FIG. 4 was charged with scrap graphite in themanner described above. The scrap graphite was electrolysed continuouslyfor a period of 144 hours without sparging. The rate at which thegraphite fell through the slits 29 fell gradually from an initial rateof 400 grammes per hour to 140 grammes per hour after 144 hours. Theexperiment was repeated using similar conditions but also with spargingas in Example 1. The initial rate of graphite falling through the slits29 was 1300 grams per hour and this was reduced to 600 grams per hourafter 144 hours. Thus the fall out rate of graphite was higher at alltimes using sparging.

EXAMPLE 3

A piece of scrap graphite was treated electrolytically as in Examples 1and 2 without sparging. The acid employed as electrolyte was saturatedwith NO₂ gas. After applying the electrical current for an hour thegraphite had not started to break down at all because of the presence ofthe NO₂ The acid was then sparged with air as in Example 1 to remove thesaturated NO₂ and the electrical current was applied again. This timethe graphite piece began to break down immediately. This illustratesthat NO₂ saturation stops the electrolytic reaction and that sparging ishighly beneficial to remove the NO₂ and promote the reaction.

I claim:
 1. A method of treating a body of conducting material electrolytically which comprises the steps of locating the material to be treated in an insulating liner received within an electrically conducting frame, said frame and said liner being inserted in an aqueous oxidizing electrolyte, said liner including a base having perforations to allow said electrolyte to contact said material to be treated, coupling said frame and said material to be treated to opposed poles of a source of electric current and passing an electric current through the electrolyte and said material to be treated and sparging the electrolyte with gas in a region where said electrolyte is in contact with said material to be treated, the gas being supplied by a plurality of pipes extending across the electrolyte at a location beneath the perforated base of said insulating liner, the pipes each having a plurality of gas outlet holes directed towards said material to be treated.
 2. The method according to claim 1 wherein said material to be treated comprises a porous mass including scrap graphite to be separated from metal adhered thereto.
 3. The method according to claim 2 wherein said metal comprises one or more hazardous elements including uranium and/or plutonium.
 4. The method according to claim 1 wherein the electric current is applied between terminals connected to said frame and one or more electrical connections to the material to be treated, said insulating liner providing electrical insulation between said frame and said material to be treated. 